Enabling a performance measurement in a packet-switched communication network

ABSTRACT

A method for enabling a performance measurement on packet flow transmitted through a communication network. A marking value is periodically switched in the packets with a marking period Tm. The packet flow is then divided into blocks of duration Ts (synchronization period). Each synchronization period comprises an integer number of marking periods. Two or more measurement points on the path of the packet flow may provide a performance parameter for each marking period and associate thereto a synchronization information generated based on their local clocks and relating to the synchronization period containing the marking period to which the performance parameter relates; and a sequence information indicating the marking period&#39;s position within the synchronization period. A management server may identify performance parameters provided by different measurement points and relating to a same marking period based on the synchronization information and the sequence information.

TECHNICAL FIELD

The present invention relates to the field of communication networks. Inparticular, the present invention relates to a method for enabling aperformance measurement in a packet-switched communication network, andto a packet-switched network configured to implement such method.

BACKGROUND ART

In a packet-switched communication network, packet flows are transmittedfrom source nodes to destination nodes through possible intermediatenodes. Exemplary packet-switched networks are IP (Internet Protocol)networks, Ethernet networks and MPLS (Multi-Protocol Label Switching)networks.

Packets not always reach their destination nodes, i.e. they may be lostduring transmission through the network. Packet loss is due to differentreasons. For instance, a node or link may fail, or packets may bediscarded by a node due to a congestion of its ports. Also, packets maybe discarded by a node since they contain bit errors.

Moreover, each packet is transmitted at a transmission time by thesource node and is received at a reception time by the destination node.The time elapsing between transmission time and reception time istypically called “one-way delay”. The one-way delay of a packet mainlydepends on the number of possible intermediate nodes crossed by thepacket from source to destination, the permanence time of the packet ateach node and the propagation time along the links.

Furthermore, packets may have different one-way delays. The differencebetween the one-way delays of two packets of a same packet flow istermed “interarrival jitter” (or, briefly, “jitter”).

When a communication service (in particular, a real-time voice or dataservice such as call, conference call, video conference, etc.) isprovided by means of a packet-switched network, a performancemeasurement in terms of packet loss, one-way delay and/or jitter onpacket flows carrying the service provides an indication of the qualityof service (QoS) perceived by the end users of the service. In addition,packet loss and high delay/jitter may require retransmission and thenreduce the efficiency of the communication network. Therefore, measuringpacket loss, one-way delay and/or jitter of packet flows in acommunication network is of particular interest for network operators.

WO 2010/072251, in the name of the same Applicant, discloses a methodfor measuring packet loss on a packet flow which uses an alternatemarking technique, whereby each packet of the packet flow to be measuredcomprises a marking bit. Upon transmission of the packet flow, the valueof the marking bit applied to the packets is periodically switched everyTm (also termed “marking period”) between a first value (e.g. “1”) and asecond value (e.g. “0”). For each marking period, the transmitting nodeand the receiving node provide a transmission counter and a receptioncounter indicating the number of packets transmitted and received duringthat marking period, respectively. A management server uses thetransmission counter and reception counter relating to the same markingperiod for calculating the packet loss for that marking period.

WO 2011/079857, in the name of the same Applicant, discloses a methodfor performing time measurements (one-way delay and/or one-way jitter)which uses the alternate marking described above. In particular, foreach marking period, the transmitting node and the receiving nodeprovide a transmission timestamp and a reception timestamp indicatingthe transmission time and the reception time of one or more samplepackets transmitted and received during that marking period,respectively. A management server uses the transmission timestamps andreception timestamps relating to the same marking period for calculatinge.g. the one-way delay for each sample packet of that marking period.

WO 2013/174417, in the name of the same Applicant, discloses a methodfor performing average time measurements which uses the alternatemarking technique described above. In particular, for each markingperiod, two measurement points on the path of the packet flow providerespective medium timestamps, each one indicating the medium receptiontime of the packets received at the measurement point during thatmarking period. A management server uses the medium timestamps providedby the measurement points and relating to the same marking period forcalculating e.g. a one-way medium delay for that marking period.

WO 2015/090364, in the name of the same Applicant, discloses a methodfor performing time measurements on a packet flow which provides fordividing the packet flow into alternating blocks (by way of example, andnot of limitation, using the alternating marking technique describedabove) and, for each block, marking a number of packets as samplepackets to be subjected to individual time measurements. The sampling isapplied so that at least a minimum time lapses between consecutivesample packets, which is shorter than a marking period but is longenough to prevent possible reception sequence errors involvingconsecutive sample packets. Two measurement points on the path of thepacket flow provide timestamps for each sample packet. A managementserver identifies the timestamps relating to sample packets transmittedduring a certain marking period and then may calculate e.g. a one-waydelay for each sample packet.

As known, QUIC (Quick UDP Internet Connections) is a transport layer(layer 4) network protocol designed to support multiplexed connectionsbetween two endpoints over User Datagram Protocol (UDP).

B. Trammel et al.: Internet draft “The addition of a Spin Bit to theQUIC Transport Protocol draft-trammel-quic-spin-01”, Dec. 13, 2017describes the addition of a so-called “latency spin bit” (or, briefly,“spin bit”) in the QUIC header, which allows RTT measurements on twocounter-propagating packet flows exchanged between two nodes. Accordingto the Internet draft, both nodes (also termed “client” and “server”)initially transmit the respective packets with the value of their spinbits set to 0. The client starts an RTT measurement by setting the valueof its spin bit to 1. This change of spin bit value may be seen as anedge in the spin bit signal transmitted from client to server. As theserver receives such edge, it changes the value of its own spin bit from0 to 1. This way, the server substantially reflects the edge of the spinbit signal back to the client. As the client receives the reflected edgeof the spin bit signal from the server, it switches the value of itsspin bit back to 0. This may be seen as another edge in the spin bitsignal transmitted from client to server, which is received at theserver and reflected back to the client as described above. A rough RTTmay then be measured at any intermediate measurement point placedbetween client and server, as the duration of a spin bit period, namelyof the time lapsing between passage in a same direction (e.g. fromclient to server) of two consecutive edges of the spin bit signal.

SUMMARY OF THE INVENTION

In order to perform a packet loss or a time measurement according toanyone of WO 2010/072251, WO 2011/079857, WO 2013/174417 or WO2015/090364, the management server shall properly identify countersand/or (average) timestamps relating to each marking period as providedby the nodes or measurement points involved in the performancemeasurement.

The alternate marking technique advantageously allows the managementserver to properly identify counters and/or timestamps relating to asame marking period, even if the local clocks of the two or more nodesor measurement points from which the counters and/or timestamps aregathered are not exactly synchronized. In the present description and inthe claims, the expression “local clock” will designate a clock signalavailable at a node of the communication network. The local clock of anode may be either generated by the node itself in an autonomous way(for example by means of a local oscillator located at the node), or maybe recovered by the node (for example by means of a clock recoverysystem) starting from a signal received from a further entity (by way ofnot limiting example, another network node, the management server, a GPSsystem, etc.).

In order to prevent the management server from erroneously mixing upcounters and/or timestamps relating to different marking periodsassociated with a same marking value, the local clocks at the nodes ormeasurement points providing the counters and/or timestamps must besynchronized with a precision not less than 50% Tm (Tm being the markingperiod), reduced by the maximum propagation delay between the two nodesor measurement points, namely:DC<(Tm/2)−Dmax  [1]where DC is the difference between the local clocks of the nodes ormeasurement points involved in the performance measurement, Tm is themarking period and Dmax is the maximum propagation delay between the twonodes or measurement points.

In order to simplify the calculation (Dmax is difficult to estimate), atime error ET may be defined as the sum of DC and Dmax. Hence, equation[1] may be rewritten as:ET<Tm/2  [2]

If Tm is set equal to e.g. 5 minutes, Tm/2 is 2.5 minutes, which is farabove the typical values of the time error ET in packet switchedcommunication networks. In this case, therefore, the management serverhas no difficulty to properly identify counters and/or timestampsrelating to a same marking period, and accordingly provides correctperformance measurements.

In some cases, however, the marking period Tm may be so short that Tm/2becomes lower than the typical values of the time error ET in packetswitched communication networks. This may be the case, for instance, ifperformance measurements with a very fine granularity are needed, e.g.for the purpose of implementing very fast failure recovery mechanisms.Alternatively, a very short marking period Tm may be the consequence ofthe fact that the alternate marking is implemented according to theabove described spin bit technique provided by QUIC protocol. In thisparticular case, indeed, the marking period Tm is determined by the RTT(round trip time) between client and server and may be as short as fewmilliseconds. In these cases, the management server could likelyerroneously mix up counters and/or timestamps relating to differentmarking periods associated with a same marking value, and accordinglyprovide incorrect performance measurements.

In view of the above, the Applicant has tackled the problem of providinga method for enabling a performance measurement in a packet-switchedcommunication network implementing an alternate marking technique, whichallows providing performance measurements with a very fine granularityand which, at the same time, allows proper identification of countersand/or timestamps relating to a same marking period.

In the following description and in the claims, the expression“performing a performance measurement in a packet-switched communicationnetwork” will designate an operation of measuring a packet loss and/or adelay and/or a jitter undergone by packets of a packet flow transmittedthrough the packet-switched communication network.

Further, in the following description and in the claims, the expression“enabling a performance measurement in a packet-switched communicationnetwork” will designate an operation of marking and/or conditioningpackets transmitted by a node of the packet-switched communicationnetwork in such a way that a performance measurement can be made,possibly at intermediate nodes.

According to embodiments of the present invention, the above problem issolved by a method wherein, upon transmission of the packet flow to bemeasured, a marking value comprised in the packets of the packet flow isperiodically switched between at least two alternative marking valueswith a certain marking period Tm. The flow of marked packets is alsodivided into blocks having a duration equal to a synchronization periodTs, where the synchronization period Ts contains an integer number ofmarking periods Tm.

Two or more measurement points may be provided on the path of the flowof marked packets divided in blocks. Each measurement point may providea plurality of performance parameters, each performance parameterrelating to packets transmitted during a respective marking period. Eachmeasurement point may also associate each performance parameter with arespective identification label comprising (i) a locally-generatedsynchronization information relating to the synchronization period Tsthat contains the marking period Tm to which the performance parameterrelates and (ii) a sequence information indicating the position of thatmarking period Tm within the synchronization period Ts. In order toperform the performance measurement, the performance parameters providedby the measurement points and relating to a same marking period Tm areidentified based on the synchronization information and the sequenceinformation in the identification labels. Then, the performancemeasurement may be performed based on the identified performanceparameters that relate to the same marking period Tm. It shall benoticed that the measurement points may be implemented and operated byan entity other than the entity managing the marking of the packets.Further, the division into blocks of the marked packets may be managedeither by the entity performing the marking or by the entity operatingone of the measurement points, or even by a further block-divisionentity arranged, on the flow of marked packets, between the markingentity and a measurement point.

Advantageously, such method is capable of enabling performancemeasurements with a very fine granularity and, at the same time, allowsproper identification of performance parameters (e.g. counters and/ortimestamps) relating to a same marking period.

While indeed the measurement granularity is given by the marking periodTm, the capability of properly identifying performance parameters (e.g.counters and/or timestamps) relating to a same marking period depends onthe synchronization period Ts, according to equations similar toequations [1] and [2] above. Hence, the marking period Tm may be chosenshort enough to obtain the desired measurement granularity, taking intoaccount the available computational resources (the computational effortin general increases as the marking period Tm decreases). On the otherhand, the synchronization period Ts may be chosen long enough to fulfilthe conditions set forth in equations [1] and [2] above, taking intoaccount the synchronization properties of the local clocks in thecommunication network and the typical propagation delays.

According to a first aspect, the present invention provides a method forenabling a performance measurement on a flow of packets transmittedthrough a packet-switched communication network, the flow of packetscomprising a marking value periodically switched between at least twoalternative marking values with a marking period, the method comprising:

-   a) dividing the flow of marked packets into blocks, each block    having a duration equal to a synchronization period, the    synchronization period comprising an integer number N of marking    periods.

Preferably, the marking value is switched upon occurrence of a certaincondition, the condition comprising one of:

-   -   detection of a change of marking value in a further flow of        packets counter-propagating to the flow of packets, the duration        of the marking period being variable; and    -   expiry of a timer counting a certain duration of the marking        periods.

Preferably, step a) comprises dividing the flow of marked packets intoblocks by periodically switching a synchronization value comprised inthe packets of flow of packets between at least two alternativesynchronization values with the synchronization period.

Step a) may be performed on the packets substantially at the same timeas the periodical switching of the marking value or, alternatively, stepa) is performed on the packets after the periodical switching of themarking value.

According to a preferred embodiment, at step a) the synchronizationperiod comprises an even integer number N of marking periods.

Preferably, step a) comprises, upon expiration of a synchronizationperiod, checking whether the expiration of the synchronization periodcoincides with the end of a marking period and, in the negative, waitingfor the end of the ongoing marking period before determining that a newsynchronization period is to be started.

According to a second aspect, the present invention provides a methodfor performing a performance measurement on a flow of packetstransmitted through a packet-switched communication network, the methodcomprising the steps of the method as set forth above and:

-   b) at each one of at least two measurement points provided on the    path of the divided flow of marked packets, providing a plurality of    performance parameters, each performance parameter relating to    packets transmitted during a respective marking period, and    associating each performance parameter with a respective    identification label comprising:    -   a synchronization information relating to the synchronization        period containing the marking period to which that performance        parameter relates, the synchronization information being        generated by the measurement point based on its local clock; and    -   a sequence information indicating the position of the marking        period within the synchronization period.

According to a preferred embodiment, the method further comprises, foreach marking period, marking as a sample packet a single packet of saidflow of marked packets and, at step b), providing a plurality ofperformance parameters, each performance parameter relating to packetstransmitted during a respective marking period, comprises providing aperformance parameter relating to said sample packet.

Preferably, at step b) the synchronization information relating to thesynchronization period containing the marking period to which theperformance parameter relates comprises one of:

-   -   a start time of the synchronization period as indicated by the        local clock of the measurement point;    -   a start time of the synchronization period as indicated by the        local clock of the measurement point and approximated to the        closest integer multiple of the duration of the synchronization        period; and    -   a counter indicating the position of the synchronization period        within the sequence of synchronization periods.

Preferably, at step b) the sequence information comprises a sequencenumber n=1, 2, . . . N indicating the position of the marking periodwithin the synchronization period.

Preferably, the method further comprises:

-   c) identifying performance parameters provided by the at least two    measurement points and relating to a same marking period based on    the synchronization information and the sequence information; and-   d) performing the performance measurement based on the identified    performance parameters provided by the at least two measurement    points and relating to said same marking period.

Preferably, step c) comprises identifying the performance parametersprovided by the at least two measurement points and relating to the samemarking period as those whose associated identification labels compriseboth synchronization information relating to a same synchronizationperiod and sequence information indicative of a same marking periodposition within the same synchronization period.

According to an embodiment, step c) comprises:

-   -   a first matching step comprising, amongst all the received and        stored performance parameters, identifying at least two sets of        N performance parameters which relate to the number N of marking        periods contained within a same synchronization period, based on        the synchronization information comprised in the associated        identification labels; and    -   a second matching step comprising, amongst the at least two sets        of N performance parameters which relate to the number N of        marking periods contained within the same synchronization        period, identifying at least two performance parameters relating        to the same marking period, based on the sequence information        comprised in the associated identification labels.

According to a third aspect, the present invention provides a node for apacket-switched communication network, said node being configured totransmit a flow of packets through the packet-switched communicationnetwork, the flow of packets comprising a marking value periodicallyswitched between at least two alternative marking values with a markingperiod, the node being configured to:

-   -   divide the flow of marked packets into blocks, each block having        a duration equal to a synchronization period, the        synchronization period comprising an integer number N of marking        periods.

According to a fourth aspect, the present invention provides ameasurement point for a packet-switched communication network, themeasurement point being arranged on the path of a flow of packetstransmitted through said packet-switched communication network, the flowof packets comprising a marking value periodically switched between atleast two alternative marking values with a marking period, the flow ofmarked packets being divided into blocks, each block having a durationequal to a synchronization period, the synchronization period comprisingan integer number N of marking periods, the measurement point beingconfigured to:

-   -   provide a plurality of performance parameters, each performance        parameter relating to packets transmitted during a respective        marking period, and    -   associating each performance parameter with a respective        identification label comprising:    -   a synchronization information relating to the synchronization        period containing the marking period to which the performance        parameter relates, the synchronization information being        generated by the measurement point based on its local clock; and    -   a sequence information indicating the position of the marking        period within the synchronization period.

According to a fifth aspect, the present invention provides apacket-switched communication network comprising a node as set forthabove and at least two measurement points as set forth above.

Preferably, the packet-switched communication network further comprisesa management server configured to:

-   -   identify performance parameters provided by the at least two        measurement points and relating to a same marking period based        on the synchronization information and the sequence information;        and    -   perform the performance measurement based on the identified        performance parameters provided by the at least two measurement        points and relating to said same marking period.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become clearer from the following detaileddescription, given by way of example and not of limitation, to be readwith reference to the accompanying drawings, wherein:

FIG. 1 schematically shows a packet-switched communication network inwhich the method for enabling performance measurements according toembodiments of the present invention is implemented;

FIG. 2 schematically shows the structure of a packet transmitted throughthe communication network of FIG. 1 , according to embodiments of thepresent invention;

FIG. 3 schematically shows the structure of a packet flow subjected to afirst and second marking, according to an embodiment of the presentinvention;

FIG. 4 is a flow chart of the operation of a node of the communicationnetwork shown in FIG. 1 , according to an embodiment of the presentinvention;

FIG. 5 is a flow chart of the operation of a measurement point of thecommunication network shown in FIG. 1 , according to an embodiment ofthe present invention; and

FIG. 6 is a flow chart of the operation of the management server of thecommunication network shown in FIG. 1 , according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 schematically shows a packet-switched communication network 100configured to enable performance measurements according to an embodimentof the present invention.

The communication network 100 comprises a plurality of nodesreciprocally interconnected by physical links according to any knowntopology, including two nodes 1 and 2 shown in FIG. 1 . The nodes 1 and2 may be connected by a single physical link or by the concatenation ofseveral physical links and intermediate nodes (not shown in thedrawings). The communication network 100 may be for instance an IPnetwork.

The node 1 is configured to transmit packets Pk to the node 2, asschematically depicted in FIG. 1 . The packets Pk may belong to a samepacket flow (namely, they may all have a same source address and a samedestination address) or to different packet flows whose paths areoverlapping between the nodes 1 and 2.

Two or more measurement points 10, 20 are preferably implemented on thepath of the packets Pk. Both measurement points 10, 20 may be located atintermediate positions of the path of the packets Pk from the node 1 tothe node 2, as schematically depicted in FIG. 1 . Alternatively, atleast one of the measurement points 10, 20 may be implemented at one ofthe nodes 1 or 2.

The measurement points 10, 20 preferably cooperate with a managementserver 30 configured to perform management tasks on the communicationnetwork 100, including performance measurements.

The packets Pk are formatted according to a certain network protocol. Byway of non limiting example, the network protocol may be the abovementioned QUIC protocol.

As schematically depicted in FIG. 2 , each packet Pk comprises a payloadPL and a header H. The payload PL comprises user data. The header H ofeach packet Pk is formatted according to the network protocol supportedby the network 100 and comprises packet forwarding information (notshown in FIG. 2 ), namely information allowing the nodes of the network100 to properly handle the packets Pk so that they reach theirdestination nodes.

According to embodiments of the present invention, the header H of eachpacket Pk preferably also comprises a marking field MF and asynchronization field SF.

The marking field MF comprises one or more bits, preferably a singlebit. The marking field MF may be set to anyone of two different markingvalues MA, MB (e.g. “1” and “0”, in case of a single bit marking field)in order to implement a first alternate marking on the packet flow PF,as it will be described in detail herein after.

Also the synchronization field SF comprises one or more bits, preferablya single bit. The synchronization field SF may be set to anyone of twodifferent synchronization values SA, SB (e.g. “1” and “0”, in case of asingle bit synchronization field) in order to implement a secondalternate marking on the same packet flow PF, as it will be described indetail herein after.

Though according to the embodiment shown in FIG. 2 the marking field MFand the synchronization field SF are both part of the header H, this isnot limiting. For example, if the packet Pk comprises multiple headerscorresponding to different network layers, the marking field MF and thesynchronization field SF may be comprised in different headers of thepacket Pk.

As mentioned above, a first alternate marking is preferably implementedon the packets Pk. Such first alternate marking may be implemented bythe node 1.

In particular, the node 1 preferably writes in the marking field MF ofeach packet Pk to be transmitted a marking value, which is alternatelyswitched between the two marking values MA and MB, as schematicallydepicted in FIG. 3 . The time lapsing between two consecutive switchingof the marking value is termed herein after “marking period Tm”. Hence,during a marking period Tm, the node 1 transmits to the node 2 packetsPk having their marking field MF set to one of the values MA, MB, e.g.MA. Then, upon expiration of the marking period Tm, the node 1 switchesthe marking value from MA to MB, so that during the subsequent markingperiod Tm it will transmit to the node 2 packets Pk having their markingfield set to MB. And so on.

The node 1 preferably switches the marking value from MA to MB or viceversa upon occurrence of a certain condition. Such condition may includefor instance detection of a change of the marking value in the packetsreceived from the node 2, as it occurs e.g. when the above describedspin bit technique of the QUIC protocol is applied. This results inmarking periods Tm with variable duration, depending on the round-triptime (RTT) between the nodes 1 and 2. This is however not limiting. Forinstance, the node 1 may switch the marking value upon expiry of a timercounting a certain predefined duration of the marking period Tm. Suchpredefined duration may be constant, thereby providing marking periodsTm having all a same duration. Alternatively, the duration of themarking period Tm may be switched between two different values, therebyproviding marking periods Tm with a certain duration that alternate intime with marking periods with another duration.

According to embodiments of the present invention, a second alternatemarking is also preferably implemented on the packets Pk.

According to an embodiment, the second alternate marking is implementedsubstantially at the same time as the first alternate marking describedabove. According to such embodiment, the node 1 may implement both thefirst and the second alternate marking on the flow of packets Pk.

According to other embodiments, the second alternate marking isimplemented after the first alternate marking, namely on the flow ofpackets Pk already processed according to the first alternate marking asdescribed above. According to such other embodiments, the secondalternate marking may be applied by a measurement point, for instancethe measurement point 10, or at any intermediate node or point betweenthe node 1 and the measurement point 10.

In case the node 1 implements both the first and the second alternatemarking on the flow of packets Pk, the node 1 preferably writes in thesynchronization field SF of each packet Pk to be transmitted asynchronization value, which is alternately switched between the twosynchronization values SA and SB, as schematically depicted in FIG. 3 .The time lapsing between two consecutive switching of thesynchronization value is termed herein after “synchronization periodTs”. Hence, during a synchronization period Ts, the node 1 transmits tothe node 2 packets Pk having their synchronization field SF set to oneof the values SA, SB, e.g. SA. Then, upon expiration of thesynchronization period, the node 1 switches the synchronization valuefrom SA to SB, so that during the subsequent synchronization period Tsit will transmit to the node 2 packets Pk having their synchronizationfield set to SB. And so on.

Preferably, the synchronization period Ts has a predefined, fixedduration. The synchronization period Ts is preferably higher than 1minute, for instance 5 minutes. The synchronization period Ts preferablycontains an integer number N of marking periods Tm, namely Ts=N×Tm. Morepreferably, the synchronization period Ts contains an even integernumber N of marking periods Tm. This way, advantageously, all thesynchronization periods start with a marking period having the sameapplicable marking value. This results in a less complex implementationof the method of the invention from the logic and computational point ofview. For instance, the marking period Tb may be equal to 1 ms and thesynchronization period Ts may be equal to 5 minutes, namely N=300 000.Preferably N is greater than 10. More preferably N is greater than 100,even more preferably greater that 1000.

The operation of the node 1 according to the embodiments wherein itimplements both the first and the second alternate marking on the flowof packets Pk will be now described in detail with reference to the flowchart of FIG. 4 .

When a performance measurement session shall be started, the node 1preferably starts a synchronization timer configured to count asynchronization period Ts of a predefined duration, e.g. 5 minutes (step40).

As the node 1 starts the synchronization timer, it preferably determinesthe synchronization value SA or SB which it will apply to the packets Pkthat it will transmit during that synchronization period Ts (step 41).

For this purpose, the node 1 for instance may store a synchronizationtable comprising a synchronization period identifier (e.g. asynchronization period start time) and an associated applicablesynchronization value. As the node 1 starts the synchronization timer,it preferably reads the current time as indicated by its local clock.The applicable synchronization value as determined at step 41 may be forinstance the one that, within the synchronization table, is associatedwith the synchronization period identifier which is closer to thecurrent time.

Then, while the local timer counting the synchronization period Ts isrunning (step 42), for each packet Pk to be transmitted the node 1preferably determines the marking value MA or MB applicable thereto asdescribed above and writes it in its marking field MF (step 43). Sincethe synchronization period Ts is selected so as to contain an integernumber of marking periods Tm, the node 1 will generally switch theapplicable marking value between MA and MB several times, before thelocal timer expires.

Further, the node 1 also preferably writes the applicablesynchronization value SA or SB as determined at step 41 in thesynchronization field SF of the packets Pk to be transmitted (step 44).The same synchronization value SA or SB will therefore be written by thenode 1 in all the packets Pk to be transmitted while the local timer isrunning.

If both steps 43 and 44 are performed by the node 1, they could beinverted, namely step 44 could be performed before step 43.

Steps 43 and 44 continue being performed until either thesynchronization timer counting the synchronization period Ts expires(step 42) or the measurement session ends (step 45).

When the synchronization timer counting the synchronization period Tsexpires, the node 1 preferably reverts to step 40, namely it restartsthe synchronization timer thereby starting a new synchronization periodTs, and determines the new applicable synchronization value (step 41).Then it continues performing steps 43 and 44 for the whole duration ofthe new synchronization period Ts.

Optionally, as depicted in FIG. 4 , when the synchronization timercounting the synchronization period Ts expires (step 42), the node 1 maycheck whether the expiration of the synchronization timer (namely, theend of the synchronization period Ts) coincides with the end of amarking period Tm and, in the negative (namely, if a marking period isongoing) it waits for the end of the ongoing marking period Tm beforereverting to step 40 (step 46). While it waits, the node 1 preferablycontinues performing the first and second marking of the packets Pk tobe transmitted (step 43 and 44 described above). In particular, while itwaits, the node 1 continues applying the synchronization value asdetermined at the last iteration of step 41.

The optional execution of step 46 is advantageous when the markingperiod Tm has a variable duration. As described above, indeed, thesynchronization period Ts is chosen so that it contains an integernumber N of marking periods Tm. However, as described above, the markingperiod Tm may have a variable duration, for instance if the firstalternate marking of step 43 is performed according to the known spinbit technique of the QUIC protocol. In this case, it can not beguaranteed a priori that every synchronization period Ts comprisesexactly an integer number N of marking periods Tm. Step 46 basicallyprovides for adjusting or adapting the duration of every singlesynchronization period Ts based on the duration of the last markingperiod contained therein, so as to prevent the latter from being splitbetween two adjacent synchronization periods. In any case, it shall benoticed that, if the synchronization period Ts is selected orders ofmagnitude higher than the marking period Tm and its typical variations(e.g. 5 minutes vs few milliseconds), step 46 will result in avariability of the actual synchronization period Ts which is orders ofmagnitude lower than the synchronization period Ts itself. Hence, thesynchronization period Ts remains substantially constant even when themarking period Tm is variable and step 46 is executed.

As mentioned above, the second marking may be performed by any of themeasurement points 10, 20, for instance the measurement point 10. Inthis case, the node 1 basically implements exclusively step 43 (firstmarking), while the other steps of the flow chart of FIG. 4 areperformed by the measurement point 10.

Each measurement point 10, 20 is preferably configured to provide aplurality of performance parameters relating to the packets Pkcontaining both a marking value MA, MB and a synchronization value SA,SB, as it will be described herein below with reference to the flowchart of FIG. 5 .

As the measurement point 10, 20 has to start a measurement session, itpreferably initializes at least one couple of performance variables(step 50). Each couple of performance variables preferably comprises afirst performance variable PV_(A) ¹⁰, PV_(A) ²⁰ relating to packets Pkcomprising the marking value MA as detected by the measurement point 10,20 and a second performance variable PV_(B) ¹⁰, PV_(B) ²⁰ relating topackets Pk comprising the marking value MB as detected by themeasurement point 10, 20.

The at least one couple of performance variables initialized by eachmeasurement point 10, 20 may comprise for instance:

-   -   a first counter variable C_(A) ¹⁰, C_(A) ²⁰ counting the number        of packets Pk comprising the marking value MA and a second        counter variable C_(B) ¹⁰, C_(B) ²⁰ counting the number of        packets Pk comprising the marking value MB; and/or    -   a first timestamp variable T_(A) ¹⁰, T_(A) ²⁰ indicating the        reception time of a sample packet Pk comprising the marking        value MA and a second timestamp variable T_(B) ¹⁰, T_(B) ²⁰        indicating the reception time of a sample packet Pk comprising        the marking value MB.

As the measurement point 10, 20 receives the packets Pk (step 51), itpreferably reads the values of their marking field MF and the values oftheir synchronization field SF (step 52).

Then, based on the values read at step 52, the measurement point 10, 20preferably updates the values of the first performance variable PV_(A)¹⁰, PV_(A) ²⁰ and second performance variable PV_(B) ¹⁰, PV_(B) ²⁰ (step53). By way of non limiting example, at step 53 the measurement point10, 20 may:

-   -   increase by 1 the value of the first counter variable C_(A) ¹⁰,        C_(A) ²⁰, upon reception of each packet Pk comprising the        marking value MA and increase by one the value of the second        counter variable C_(B) ¹⁰, C_(B) ²⁰ upon reception of each        packet Pk comprising the marking value MB; and/or    -   set the value of the first timestamp variable T_(A) ¹⁰, T_(A) ²⁰        equal to the reception time (as indicated by a local clock of        the measurement point 10, 20) of a sample packet Pk comprising        the marking value MA and set the value of the second timestamp        variable T_(B) ¹⁰, T_(B) ²⁰ equal to the reception time (as        indicated by a local clock of the measurement point 10, 20) of a        sample packet Pk comprising the marking value MB.

In the latter case, a packet Pk may be identified as a sample packet bythe measurement point 10, 20 in different ways.

For instance, the packets Pk may comprise a suitable field in theirheader (also termed herein after “sampling field”) which is set by thenode 1 to a first, non sampling value (e.g. “0”) in packets Pk not to beconsidered as sample packets and to a second, sampling value (e.g. “1”)in packets Pk to be considered as sample packets. According to anadvantageous variant, the node 1 marks as sample packet a single packetPk for each marking period Tm, for example the first packet Pk to betransmitted subsequently the beginning of each marking period Tm.

If the marking period Tm is of the order of magnitude of milliseconds(e.g. when the first marking is implemented using the known spin bittechnique of the QUIC protocol as described above), marking as samplepacket a single packet Pk for each marking period Tm advantageouslyprovides a high number of sample packets. Further, providing a singlesample packet Pk for each marking period Tm advantageously minimizes therisk of reception sequence errors involving sample packets comprisingthe same marking value MA or MB. Further, if a sample packet is lost,this affects the performance measurements of a marking period Tm only,and not of the whole synchronization period Ts.

Alternatively, the measurement points 10, 20 may be configured toidentify certain packets Pk as sample packets only based on theirpositions within the flow of packets Pk. For example, the measurementpoints 10, 20 may be configured to consider as a sample packet (andhence to properly set the first timestamp variable T_(A) ¹⁰, T_(A) ²⁰ orthe second timestamp variable T_(B) ¹⁰, T_(B) ²⁰, as described above)the first packet Pk received during each marking period Tm.

Hence, at marking periods Tm during which the packets Pk comprise themarking value MA, the first performance variable PV_(A) ¹⁰, PP_(A) ²⁰ isrunning or updated, while the second performance variable PV_(B) ¹⁰,PV_(B) ²⁰ is idle (namely, its value is constant, short of receptionsequence errors between packets Pk with different marking values).Conversely, at marking periods Tm during which the packets Pk comprisethe marking value MB, the first performance variable PV_(A) ¹⁰, PV_(A)²⁰ is idle (namely, its value is constant, short of reception sequenceerrors between packets Pk with different marking values), while thesecond performance variable PV_(B) ¹⁰, PV_(B) ²⁰ is running or updated.

While updating the values of the first performance variable PV_(A) ¹⁰,PV_(A) ²⁰ and second performance variable PV_(B) ¹⁰, PV_(B) ²⁰ asdescribed above, the measurement point 10, 20 preferably provides aplurality of performance parameters PP¹⁰, PP²⁰, each performanceparameter being indicative of the behavior of the packets Pk during arespective marking period Tm (step 54). In particular, herein belowPP¹⁰(k, n), PP²⁰(k, n) with k=1, 2, 3, . . . and n=1, 2, . . . N willdesignate the performance parameter relating to the n^(th) markingperiod Tm comprised in the k^(th) synchronization period Ts since thebeginning of the measurement session.

Preferably, at step 54 the performance parameter PP¹⁰(k, n), PP²⁰(k, n)is provided by the measurement point 10, 20 after the end of the n^(th)marking period Tm, as the currently constant value of the performancevariable PV_(A) ¹⁰, PV_(A) ²⁰ or PV_(B) ¹⁰, PV_(B) ²⁰ which is currentlyidle. Hence, the performance parameter PP¹⁰(k, n), PP²⁰(k, n) relatingto each marking period Tm wherein packets Pk are marked by MA isprovided by the measurement point 10, 20 after the end of that markingperiod Tm, as the currently constant value of the second performancevariable PV_(B) ¹⁰, PV_(B) ²⁰, while the performance parameter PP¹⁰(k,n), PP²⁰(k, n) relating to each marking period Tm wherein packets Pk aremarked by MB is provided by the measurement point 10, 20 after the endof that marking period Tm, as the currently constant value of the firstperformance variable PV_(A) ¹⁰, PV_(A) ²⁰.

Preferably, the measurement point 10, 20 delays by a certain time gapthe provision of the performance parameter PP¹⁰(k, n), PP²⁰(k, n)relative to the end of the n^(th) marking period Tm. This guaranteesthat the considered performance variable is actually idle (namely, ithas a currently constant value) when it is used for providing theperformance parameter PP¹⁰(k, n), PP²⁰(k, n), even in case of receptionsequence errors between packets Pk at the boundary between the n^(th)marking period and the (n+1)^(th) marking period. Such time gap ispreferably lower than Tm/2.

Hence, by way of non limiting example, at step 54 the measurement point10, 20 may:

-   -   after the end of each marking period Tm wherein packets are        marked by MA, provide a counter C¹⁰(k, n), C²⁰(k, n) as the        currently constant value of the first counter variable C_(A) ¹⁰,        C_(A) ²⁰ and, after the end of each marking period Tm wherein        packets are marked by MB, provide a counter C¹⁰(k, n), C²⁰(k, n)        as the currently constant value of the second counter variable        C_(B) ¹⁰, C_(B) ²⁰; and/or    -   after the end of each marking period Tm wherein packets are        marked by MA, provide a timestamp T¹⁰(k, n), T²⁰(k, n) as the        currently constant value of the first timestamp variable T_(A)        ¹⁰, T_(A) ²⁰ and, after the end of each marking period Tm        wherein packets are marked by MB, provide a timestamp T¹⁰(k, n),        T²⁰(k, n) as the currently constant value of the second        timestamp variable T_(B) ¹⁰, T_(B) ²⁰.

According to embodiments of the present invention, the measurement point10, 20 preferably associates each performance parameter PP¹⁰(k, n),PP²⁰(k, n) provided at step 54 with a respective identification labelIL¹⁰(k, n), IL²⁰(k, n) (step 55). The identification label IL¹⁰(k, n),IL²⁰(k, n) associated with each performance parameter PP¹⁰(k, n),PP²⁰(k, n) preferably comprises:

-   -   a synchronization information relating to the k^(th)        synchronization period Ts; and    -   a sequence information indicative of the position the n^(th)        marking period Tm within the k^(th) synchronization period Ts.

Preferably, the synchronization information relating to the k^(th)synchronization period Ts is generated by the measurement point 10, 20based on (i) the synchronization values SA, SB comprised in the receivedpackets Pk and (ii) a local clock of the measurement point 10, 20.

For example, the synchronization information relating to the k^(th)synchronization period Ts may comprise the start time t_(start)(k) ofthe k_(th) synchronization period Ts as indicated by the local clock ofthe measurement point 10, 20. The measurement point 10, 20 preferablydetermines that a new synchronization period Ts is starting as itdetects a change of the synchronization value comprised in the receivedpackets Pk (e.g. from SA to SB or vice versa). Upon detection of thebeginning of a new synchronization period Ts, the measurement point 10,20 preferably reads the current time as indicated by its local clock,thereby obtaining the start time t_(start)(k) of the new synchronizationperiod Ts.

Optionally, at step 55 the measurement point 10, 20 may approximate thestart time t_(start)(k) of the k^(th) synchronization period Ts asindicated by its own local clock to the closest integer multiple of Ts,t′_(start)(k), and use such approximated value as synchronizationinformation within the identification label IL¹⁰(k, n), IL²⁰(k, n). Forexample, if the synchronization period Ts is 5 minutes and the starttimes of the k^(th) synchronization period Ts as indicated by the localclock of the measurement point 10, 20 is t_(start)(k)=31:50, themeasurement point 10, 20 may approximate it to the closest integermultiple of 5 minutes, namely t′_(start)(k)=30:00.

According to other variants, the synchronization information relating tothe k^(th) synchronization period Ts may comprise the value of a counterC(k) indicating the position of the k^(th) synchronization period Tswithin the sequence of the synchronization periods.

Preferably, the value of the counter C(k) is in any case derived fromthe current time as indicated by the local clock of the measurementpoint 10, 20.

As to the sequence information, it preferably comprises a sequencenumber n=1, 2, . . . N indicating the position of the marking period Tmwithin the synchronization period Ts. The measurement point 10, 20 mayprovide the correct value of n for example by implementing a variablewhich is reset to 0 at the beginning of each synchronization period Tsdetermined as described above and is increased by 1 each time themeasurement point 10, 20 determines the beginning of a new markingperiod Tm as a change of the marking value (e.g. from MA to MB or viceversa) comprised in the received packets Pk. The current value of suchvariable after the end of each marking period Tm may be used as thesequence information to be associated with the performance parameterPP¹⁰(k, n), PP²⁰(k, n) relating to that marking period Tm.

Hence, the identification label IL¹⁰(k, n), IL²⁰(k, n) associated withthe performance parameter PP¹⁰(k, n), PP²⁰(k, n) relating to the n^(th)marking period Tm of the k^(th) synchronization period Ts since thebeginning of the performance measurement for example may be(t_(start)(k); n) or (t′_(start)(k); n).

Then, the measurement point 10, 20 preferably sends to the managementserver 30 the plurality of performance parameters PP¹⁰(k, n), PP²⁰(k, n)as provided at step 54 with the associated identification labels IL¹⁰(k,n), IL²⁰(k, n) as provided at step 55 (step 56).

At step 56, the measurement point 10, 20 may send to the managementserver 30 each single performance parameter PP¹⁰(k, n), PP²⁰(k, n) withits identification label IL¹⁰(k, n), IL²⁰(k, n) as soon as they havebeen generated at steps 54 and 55. Hence, the measurement point 10, 20sends to the management server 30 a performance parameter every markingperiod Tm.

Alternatively, the measurement point 10, 20 may simultaneously send tothe management server 30 a plurality of performance parameters PP¹⁰(k,n), PP²⁰(k, n) (with the respective identification labels) relating to acorresponding plurality of consecutive marking periods Tm. For example,at the end of each synchronization period Ts, the measurement point 10,20 may simultaneously send to the management server 30 the N performanceparameters PP¹⁰(k, n), PP²⁰(k, n) (n=1, 2, . . . N) relating to the Nmarking periods Tm contained within the last synchronization period Ts.

The management server 30 then receives a plurality of performanceparameters PP¹⁰(k, n) with the respective identification labels IL¹⁰(k,n) from the measurement point 10 and a plurality of performanceparameters PP²⁰(k, n) with the respective identification labels IL²⁰(k,n) from the measurement point 20 and perform performance measurementsbased on them.

In particular, with reference to the flow chart of FIG. 6 , as themanagement server 30 starts receiving the performance parameters PP¹⁰(k,n) and PP²⁰(k, n) with the respective identification labels IL¹⁰(k, n)and IL²⁰(k, n) from the measurements points 10 and 20 (step 60),respectively, it preferably stores them locally (step 61).

Then, the management server 30 preferably matches each performanceparameter PP¹⁰(k, n) received from the measurement point 10 with arespective performance parameter PP²⁰(k, n) received from themeasurement point 20 and relating to the same marking period Tm, basedon the content of their identification labels IL¹⁰(k, n) and IL²⁰(k, n).

In particular, the management server 30 preferably identifies asperformance parameters relating to a same marking period Tm those whoseassociated identification labels comprise both synchronizationinformation relating to the same synchronization period Ts and sequenceinformation indicative of the same marking period position within thesynchronization period Ts.

Such matching operation preferably is a two-step operation.

During a first matching step 62, the management server 30 preferablyidentifies, amongst all the received and stored performance parameters,the two sets of N performance parameters PP¹⁰(k, n) and PP²⁰(k, n) (n=1,2, . . . N) which relate to the N marking periods Tm contained within asame synchronization period Ts, based on the synchronization informationcomprised in the associated identification labels IL¹⁰(k, n) and IL²⁰(k,n).

Since, as described above, each measurement point 10, 20 generates thesynchronization information based on its own local clock, thesynchronization information comprised in the identification labelsIL¹⁰(k, n) and IL²⁰(k, n) associated with performance parameters PP¹⁰(k,n) and PP²⁰(k, n) relating to a same synchronization period Ts may havedifferent values. For instance, if the synchronization informationrelating to the k^(th) synchronization period Ts comprises its starttime t_(start)(k) determined as described above, the start time asdetermined by the measurement point 10 may be different from the starttime as determined by the measurement point 20, due to both thepropagation delay of packets Pk between the measurement point 10 and themeasurement point 20 and the difference between their local clocks.

It may be however appreciated that, consistently with the aboveequations [1] and [2], the management server 30 is capable ofunambiguously recognizing the start times determined by the measurementpoints 10 and 20 for a same synchronization period if:DC<(Ts/2)−Dmax  [3]where DC is the difference between the local clocks of the measurementpoints 10, 20 and Dmax is the maximum propagation delay between themeasurement points 10, 20 or, equivalently, if:ET<Ts/2  [4]where ET is the time error defined as the sum of DC and Dmax.

If the synchronization period Ts is set equal to e.g. 5 minutes, Ts/2 is2.5 minutes, which is far above the typical values of the time error ETin packet switched communication networks. Hence, at step 62 themanagement server 30 has no difficulty to properly identify, amongst allthe performance parameters PP¹⁰(k, n) and PP²⁰(k, n) received from themeasurements points 10 and 20, those that relate to the N markingperiods Tm contained within a same synchronization period Ts, based onthe synchronization information comprised in the identification labelsIL¹⁰(k, n) and IL²⁰(k, n) associated thereto.

Then, during a second matching step 63, amongst all the performanceparameters PP¹⁰(k, n) and PP²⁰(k, n) received from the measurementspoints 10 and 20 and relating to the N marking periods Tm containedwithin the synchronization period Ts identified at step 62, themanagement server 30 preferably identifies the two performanceparameters PP¹⁰(k, n) and PP²⁰(k, n) relating to the same marking periodTm, based on the sequence information comprised in the associatedidentification labels IL¹⁰(k, n) and IL²⁰(k, n).

In particular, at step 63 the management server 30 preferably identifiesthe two performance parameters PP¹⁰(k, n) and PP²⁰(k, n) relating to thesame marking period Tm as those whose associated identification labelsIL¹⁰(k, n) and IL²⁰(k, n) comprise the same sequence information, e.g.the same sequence number.

Then, the management server 30 preferably provides a performancemeasurement for each marking period Tm, using the two performanceparameters PP¹⁰(k, n) and PP²⁰(k, n) relating to the same marking periodTm as identified at step 63 (step 64).

By way of non limiting example, at step 64 the management server 30 maycalculate, for each marking period Tm:

-   -   a packet loss PL(k,n) as a difference between the counter        C¹⁰(k, n) provided by the measurement point 10 and the counter        C²⁰(k, n) provided by the measurement point 20; and/or    -   a one-way delay OWD(k,n) as a difference between the timestamp        T²⁰(k, n) provided by the measurement point 20 and the timestamp        T¹⁰(k, n) provided by the measurement point 10.

Other types of measurements are possible, such as average timemeasurements and/or jitter measurements.

It may be appreciated that the granularity of the performancemeasurements provided at step 64 is equal to the marking period Tm.Hence, if the marking period Tm is very short (e.g. few milliseconds),either because the first marking is implemented based on the spin bittechnique of the QUIC protocol, or because a very low value of Tm isexpressly chosen for the purpose of e.g. implementing very fast failurerecovery mechanisms, a performance measurement with a very finegranularity is advantageously provided.

Despite such fine measurement granularity, as described above themanagement server 30 is nonetheless capable of properly identifying theperformance parameters that relate to the same marking period Tm. Thisis due to the fact that identification of the performance parametersrelating to the same, specific marking period Tm is a two-step processwhose first step provides for identification of all the performanceparameters relating to a same synchronization period Ts. By selecting Tsso that the conditions set forth in the above equations [3] and [4] arefulfilled, the management server 30 may properly identify all theperformance parameters relating to a same synchronization period Ts,based on synchronization information generated by the measurement pointsbased on their local clocks.

According to an advantageous variant of the present invention,performance measurements with different granularities Tm and Ts may alsobe made. For example, while the one-way delay may be measured with afiner granularity equal to the marking period Tm as described above, thepacket loss may be measured with a coarser granularity equal to thesynchronization period Ts. This may be implemented by updating thevalues of the first counter variables C_(A) ¹⁰, C_(A) ²⁰ and the secondcounter variables C_(B) ¹⁰, C_(B) ²⁰ based on the synchronization valuesSA, SB, instead of the marking values MA, MB, so that the first countervariables C_(A) ¹⁰, C_(A) ²⁰ count the number of packets Pk comprisingthe synchronization value SA, while the second counter variables C_(B)¹⁰, C_(B) ²⁰ count the number of packets Pk comprising thesynchronization value SB. According to such embodiment, each measurementpoint 10, 20 preferably provides a single performance parameter, namelya counter, for each synchronization period Ts. Each measurement pointassociates to each counter an identification label comprising only thelocally-generated synchronization information relating to thesynchronization period Ts to which the counter relates, e.g. the starttime of the synchronization period Ts as indicated by its local clock.The management server will identify counters provided by differentmeasurement points and relating to the same synchronization period Tsbased exclusively on the synchronization information comprised in theidentification labels associated with the counters received from themeasurement points.

Though in the above description reference has been made to a managementserver 30 cooperating with the measurement points 10, 20 and in chargeof performing the operations of the flow chart of FIG. 6 , this is notlimiting. According to other embodiments not shown in the drawings, theoperations of the flow chart of FIG. 6 may be carried out by any one ofthe measurement points 10, 20. In that case, the other measurement pointpreferably sends its performance parameters and associatedidentification labels to the measurement point in charge of performingthe operations of the flow chart of FIG. 6 .

Further, though according to the above description the packets Pk aresubjected to a first and second marking only, according to otherembodiments the packets Pk may be subjected to further marking with afurther period Ts' higher than the synchronization period Ts. In thiscase, only the longer period Ts' is identified based on synchronizationinformation, while both Ts and Tm are identified based on sequenceinformation.

Further, though according to the above description the division of theflow of marked packets Pk in blocks having duration Ts is implemented bya second marking nested with the first marking, this is also notlimiting. According to other variants, such division may be implementedin other ways, e.g. by using special packets to delimit the boundarybetween two consecutive blocks.

Further, though according to the above description both the firstmarking and the second marking provide for two alternative markingvalues only (SA/SB and MA/MB), this is not limiting. According to othervariants of the present invention, the first marking and/or the secondmarking may provide for N alternative marking values that are cyclicallyapplied, with N higher than 2.

The invention claimed is:
 1. A method for enabling a performancemeasurement on a flow of packets transmitted through a packet-switchedcommunication network, the flow of packets comprising a marking valueperiodically switched between at least two alternative marking valueswith a marking period, the method comprising: a) dividing the flow ofmarked packets into blocks, each block having a duration equal to asynchronization period, the synchronization period comprising an integernumber N of the marking periods, and the integer number N being greaterthan 1, including: periodically switching a synchronization valuecomprised in the packets of the packet flow between at least twoalternative synchronization values with the synchronization period, thesynchronization value being distinct from the marking value, wherein themarking value periodically switches between the at least two alternativemarking values in the marking periods within the synchronization period.2. The method according to claim 1, wherein the marking value isswitched upon occurrence of a certain condition, the conditioncomprising one of: detection of a change of a marking value in a furtherflow of packets counter-propagating to the flow of packets, the durationof the marking period being variable; and expiry of a timer counting acertain duration of the marking periods.
 3. The method according toclaim 1, wherein the synchronization value periodically switches betweenthe at least two alternative synchronization values across at least twoof the synchronization periods.
 4. The method according to claim 1,wherein: step a) is performed on the packets substantially at the sametime as the periodically switching the marking value; or step a) isperformed on the packets after the periodically switching the markingvalue.
 5. The method according to claim 1, wherein at step a) thesynchronization period comprises an even integer number N of markingperiods.
 6. The method according to claim 1, wherein step a) comprises,upon expiration of a synchronization period, checking whether theexpiration of the synchronization period coincides with an end of amarking period and, in the negative, waiting for the end of the ongoingmarking period before determining that a new synchronization period isto be started.
 7. A method for performing a performance measurement on aflow of packets transmitted through a packet-switched communicationnetwork, the method comprising: the steps of the method according toclaim 1; and b) at each one of at least two measurement points providedon a path of the divided flow of marked packets, providing a pluralityof performance parameters, each performance parameter relating topackets transmitted during a respective marking period, and associatingeach performance parameter with a respective identification labelcomprising: synchronization information relating to a synchronizationperiod containing a marking period to which the performance parameterrelates, the synchronization information being generated by themeasurement point based on its local clock; and sequence informationindicating a position of the marking period within the synchronizationperiod.
 8. The method according to claim 7, wherein at step b) thesynchronization information relating to the synchronization periodcontaining the marking period to which the performance parameter relatescomprises one of: a start time of the synchronization period asindicated by the local clock of the measurement point; a start time ofthe synchronization period as indicated by the local clock of themeasurement point and approximated to a closest integer multiple of aduration of the synchronization period; and a counter indicating aposition of the synchronization period within a sequence ofsynchronization periods.
 9. The method according to claim 7, wherein atstep b) the sequence information comprises a sequence number n=1, 2, . .. N indicating a position of the marking period within thesynchronization period.
 10. The method according to claim 7, furthercomprising, for each marking period, marking as a sample packet a singlepacket of the flow of marked packets, wherein at step b) the providingthe plurality of the performance parameters, each performance parameterrelating to the packets transmitted during the respective markingperiod, comprises providing a performance parameter relating to thesample packet.
 11. The method according to claim 7, further comprising:c) identifying performance parameters provided by the at least twomeasurement points and relating to a same marking period based on thesynchronization information and the sequence information; and d)performing the performance measurement based on the identifiedperformance parameters provided by the at least two measurement pointsand relating to the same marking period.
 12. The method according toclaim 11, wherein step c) comprises identifying the performanceparameters provided by the at least two measurement points and relatingto the same marking period as those whose associated identificationlabels comprise both synchronization information relating to a samesynchronization period and sequence information indicative of a samemarking period position within the same synchronization period.
 13. Themethod according to claim 12, wherein step c) comprises: a firstmatching step comprising, amongst all received and stored performanceparameters, identifying at least two sets of N performance parameterswhich relate to the number N of marking periods contained within a samesynchronization period based on the synchronization informationcomprised in the associated identification labels; and a second matchingstep comprising, amongst the at least two sets of N performanceparameters which relate to the number N of marking periods containedwithin the same synchronization period, identifying at least twoperformance parameters relating to the same marking period based on thesequence information comprised in the associated identification labels.14. A node for a packet-switched communication network, the node beingconfigured to transmit a flow of packets through the packet-switchedcommunication network, the flow of packets comprising a marking valueperiodically switched between at least two alternative marking valueswith a marking period, the node comprising a processor configured to:divide the flow of marked packets into blocks, each block having aduration equal to a synchronization periods, the synchronization periodcomprising an integer number N of the marking periods, and the integernumber N being greater than 1, including: periodically switching asynchronization value comprised in the packets of the packet flowbetween at least two alternative synchronization values with thesynchronization period, the synchronization value being distinct fromthe marking value, wherein the marking value periodically switchesbetween the at least two alternative marking values in the markingperiods within the synchronization period.
 15. A measurement point for apacket-switched communication network, the measurement point beingarranged on a path of a flow of packets transmitted through thepacket-switched communication network, the flow of packets comprising amarking value periodically switched between at least two alternativemarking values with a marking period, the flow of marked packets beingdivided into blocks, each block having a duration equal to asynchronization period, the synchronization period comprising an integernumber N of the marking periods, the integer number N being greater than1 including periodically switching a synchronization value comprised inthe packets of the packet flow between at least two alternativesynchronization values with the synchronization period, thesynchronization value being distinct from the marking value, and themarking value being periodically switched between the at least twoalternative marking values in the marking periods within thesynchronization period, and the measurement point comprising a processorconfigured to: provide a plurality of performance parameters, eachperformance parameter relating to packets transmitted during arespective marking period, and associating each performance parameterwith a respective identification label comprising: synchronizationinformation relating to a synchronization period containing a markingperiod to which the performance parameter relates, the synchronizationinformation being generated by the measurement point based on its localclock; and sequence information indicating the position of the markingperiod within the synchronization period.
 16. A packet-switchedcommunication network comprising: the node according to claim 14; and ameasurement point, the measurement point being arranged on a path of theflow of packets transmitted through the packet-switched communicationnetwork, the measurement point being configured to: provide a pluralityof performance parameters, each performance parameter relating topackets transmitted during a respective marking period, and associatingeach performance parameter with a respective identification labelcomprising: synchronization information relating to a synchronizationperiod containing a marking period to which the performance parameterrelates, the synchronization information being generated by themeasurement point based on its local clock; and sequence informationindicating the position of the marking period within the synchronizationperiod.
 17. The packet-switched communication network according to claim16, further comprising a management server configured to: identifyperformance parameters provided by at least two of the measurementpoints and relating to a same marking period based on thesynchronization information and the sequence information; and perform aperformance measurement based on the identified performance parametersprovided by the at least two measurement points and relating to the samemarking period.
 18. The method according to claim 1, wherein thesynchronization value is the same for the marked packets in thesynchronization period.